1. Field of the Invention
The present invention is related to micro-electronics and more particular to the field of thin film device applications such as silicon-on-insulator (SOI) structures or solar cells in particular. Furthermore, the present invention is relating to a manufacturing method of such devices.
2. Description of Related Technology
The silicon in existing semiconductor devices usually has a thickness of several hundred microns. However, the electrically active domain of a wafer is limited to its surface; in fact, less than a few microns of thickness is needed. The remaining portion of the wafer is used as substrate. Unfortunately, this excess of material causes both a rise in power consumption and a fall in the operating speed of the device. The SOI wafers incorporate an insulating layer between its very thin (less than a few microns) active domain and its much thicker substrate. The substrate is isolated and can thus no longer deteriorate the speed or efficiency of the active layer.
Silicon on insulator technology (SOI) involves the formation of a monocrystalline silicon semi-conductor layer on an insulating material such as silicon oxide.
One important application of a thin film device is the manufacturing of solar cells. Solar cells usually comprise an active surface on the top of a silicon wafer in the form of a thin film device deposited on said silicon wafer.
During the conversion of light into electrical energy, as mentioned above, only the top few microns of said top layer are really active. The major part of this silicon wafer only provides mechanical strength to the device. This function can be achieved by any other low-cost substrate compatible to the production process. The requirements for such a substrate, excepted low cost, are high temperature stability (1100° C.), matching of the thermal expansion coefficients and low impurity contents.
More generally, in the prior art, the preparation of a porous semiconductor layer on a substrate as a sacrificial layer for solar cell usually comprises several steps such as at least a porous semiconductor layer formation on an original substrate, epitaxial silicon layer deposition, device fabrication on said substrate and separation of the device from the original substrate and transfer to a foreign substrate in order to possibly re-use the original substrate. This sequence is largely illustrated in the documents U.S. Pat. No. 6,258,698 (Iwasaki et al, Canon), U.S. Pat. No. 6,211,038 (Nakagawa et al, Canon) and U.S. Pat. No. 6,326,280 (Tayanaka, Sony Corporation).
In the prior art, several methods are known to separate thin (porous) semiconductor films from a substrate. All those methods use a lift-off or peeling-off process at the end of the production chain. The drawback of these methods is that during all the process steps, parameters such as temperature, pressure and chemicals are conditioned by the resistance of the original substrate. The film separation and its transfer is the last technological step that requires preserving the high-porous characteristics of the Si layer. The fact of maintaining said porous characteristics throughout many high-temperature steps allows only a narrow processing window in terms of process temperature and porosity. Moreover in said case, the transfer is difficult to achieve properly.
In particular, a lift-off process is described in EP-A-1132952 where it is shown that a thin porous silicon film of 5 to 50 μm can be separated from the silicon substrate whereon it is deposited. In such case, the substrate can be re-used many times for getting new porous silicon films. Other possible techniques for thin film separation are ion implantation or wafer bonding techniques.
In EP-A-0993029, a method is disclosed for the production of a crystalline semiconductor film. This is done by forming a porous layer on a semiconductor substrate, lifting-off the porous layer, and either before or after the lifting-off applying a thermal annealing step such that the porous layer is at least partially recrystallized. For the lift-off step a method is disclosed in which the porous layer is attached to a ‘Hilfsträger’, which can be translated as a ‘sub-carrier’, or a ‘foreign substrate’. The porous layer is physically bonded or glued to said foreign substrate.